A switched-mode power supply must include inherent controlling functionalities that ensure controlled operation even in exceptional situations. Thinking about a battery charger for example, it is most certain that situations will occur where the input power is on, but there is no load coupled to the charger. Without control measures with some kind of limiting effects, continuously pumping electric power to the secondary side would cause the output voltage to rise above the nominal output voltage level. A short circuit at the output, on the other hand, could easily cause the output current to achieve unacceptably high values.
A very commonly used approach for limiting output voltage and output current is illustrated schematically in FIGS. 1a and 1b. A switched-mode power supply 100 comprises a primary side 101 and a secondary side 102 separated from each other by a transformer 103. A switch 104 on the primary side repeatedly switches the current flowing through a primary coil 105, which causes energy to be stored into the magnetic field of the transformer 103. A diode 106 on the secondary side only allows current to flow in one direction through a secondary coil 107. A capacitor 108 coupled across the output of the device smoothens the output voltage.
In order to monitor the output current that flows out of the switched-mode power supply, there is a small resistor 109 coupled in series with the diode 106 and the secondary coil 107. When a normal load is coupled to the output, a current of some reasonable level flows through the resistor 109 causing a voltage drop ΔV. A monitoring circuit 110 is arranged to measure the value of ΔV and to trigger some limiting action if the measured value is too high, which would indicate a short circuit at the output. As a response to an output given by the monitoring circuit 110, typically a control entity somewhere in the switched-mode power supply limits the amount of electric energy that is pumped to the transformer.
The switched-mode power supply of FIG. 1a is also adapted to monitor the output voltage. The secondary side 103 includes a series coupling of two relatively large resistors 111 and 112 coupled across the output voltage. These resistors constitute a voltage divider. A monitoring circuit 113 monitors the voltage across one of the resistors 111 and 112 (here resistor 112), which is directly proportional to the output voltage. The monitoring circuit 113 is adapted to trigger limiting action if the voltage drop across resistor 112 rises higher than a predetermined limit, which would indicate an overvoltage situation at the output.
FIG. 1b is an output voltage per output current diagram that illustrates graphically the controlling effects of the monitoring circuits 110 and 113. During normal operation, when the output current has some value in the range approximately designated as 120, the output voltage monitoring circuit 113 is active and keeps the output voltage at some predetermined level U1. If the output current tries to grow larger than a limiting value I1, the output current monitoring circuit 110 steps in to keep the output current from rising any higher.
At the leftmost part of the graph in FIG. 1b there is an area 121 that may involve some uncertainty in the operation of the switched-mode power supply. For example when a battery is almost full, it will only draw a very small current from the switched-mode power supply of a charger. In general, this tends to cause the output voltage to rise. When there is no load at all, there will be no actual output current but only some small leakage currents that together with the continuous pumping of energy from the primary cause the output voltage to assume some value, which is typically higher than U1. This may lead to harmful effects, such as unnecessarily warming up the switched-mode power supply. Additionally “overstuffing” the secondary side with electric energy when there is no load means that at the very moment when a load is connected, there will be a rushing, potentially excessively high initial current to the load before any controlling circuitry comes effective again.